Membrane bioreactor system for treating synthetic metal-working fluids and oil-based products

Abstract

A system to treat wastewater from a metal-working facility, such as an automotive manufacturing plant in a bioreactor using live microorganisms. Such wastewater contains waste fluids which are a mixture of relatively easily biodegradable fats and oils, much less easily biodegradable synthetic fluids, oils and greases, and non-biodegradable material including inorganic finely divided solids such as metal and silicon carbide particles. Such waste fluids require a hydraulic retention time (HRT) and a solids retention time (SRT) which is 10 times greater than for sewage. High quality water is separated from suspended solids which are removed from the reactor at an essentially constant rate and fed to an ultrafiltration membrane. Concentrate is recycled to the reactor, except for a bleed stream to remove solids periodically. The membranes acquire a long and effective life despite large variations in membrane flux, because of a permeate recycle which permits operation of the bioreactor at constant volume; permits flow of feed wastewater to the reactor at constant flow rate; and allows operation of the membrane modules at a relatively low pressure in a narrow range which does not damage the membranes. Such operation of the bioreactor allows one to use a reactor which is one-half the size (volume) than one which would be required with a system without a permeate recycle. Effective and long-lived operation of the membranes is obtained by filtering out (through a 140 mesh screen) all solids greater in diameter than about 106 μm. Pilot plant tests conducted with wastewater from automobile manufacturing plants over a period of more than a year provide evidence of the surprising effectiveness of the system over a prolonged period.

We claim: 
     
       1. A process for continuously biodegrading biochemically oxidizable material comprising a solids-containing mixture of easy- and difficult-to-degrade waste fluids in a wastewater stream from a metal-working plant, said process comprising, (a) providing a pretreatment zone for said wastewater, said zone having a volume sufficiently large to equalize variations in the flow rate of said wastewater to said pretreatment zone which is adapted for removal of settlable solids and skimmable free oil, and removing skimmed wastewater from said zone;   (b) removing finely divided solids injurious to membranes, from said skimmed wastewater to provide a solids-depleted wastewater feed;   (c) feeding said solids-depleted wastewater feed at an essentially constant rate of flow to a bioreaction zone maintained with an essentially constant volume of liquid therein, and, with a hydraulic retention time of at least 24 hr;   (d) aerating said biochemically oxidizable material in the presence of live microorganisms adapted to degrade said materials which are held in suspension within said reactor with a solids retention time (SRT) of at least 30 days;   (e) flowing said suspension through a membranous filtration zone at a velocity and pressure sufficient to maintain a predetermined membrane flux in said filtration zone at which flux essentially no solids are retained on the surface of membrane in said filtration zone, said membranous zone having a membrane area sufficiently large to provide a flow rate of permeate greater than said essentially constant rate of flow of said solids-depleted wastewater feed;   (f) separating permeate from a concentrate containing said solids;   (g) flowing said concentrate from said membranous filtration zone into said bioreaction zone as a concentrate recycle;   (h) removing an effluent of acceptable quality;   (i) flowing excess permeate over that removed as effluent, back to said bioreaction zone; and,   (j) periodically removing a minor proportion by volume of said concentrate recycle to modulate the solids content in said bioreaction zone.   
     
     
       2. The process of claim 1 wherein the flow of said excess permeate to said bioreaction zone is a minor proportion by volume of said permeate removed as effluent. 
     
     
       3. The process of claim 1 wherein said finely divided solids are smaller than about 106 μm in diameter. 
     
     
       4. The process of claim 2 wherein said waste fluids comprise organometallic compounds, natural, petroleum-based, synthetic and semi-synthetic fats, oils and greases used in machining operations in said metal-working plant. 
     
     
       5. The process of claim 4 wherein said effluent has at least the following specifications: Chemical Oxygen Demand, COD<450 mg/L;   Biological Oxygen Demand, BOD 5  <25 mg/L;   Total suspended solids, TSS<10 mg/L;   Total FOG<25 mg/L; and NH 3  --N<1.0 mg/L; wherein FOG represents fats, oils and greases.     
     
     
       6. The process of claim 5 wherein said membranous filtration zone is operated at a pressure in the range from 170 kPa to 1035 kPa gauge, insufficient to affect membranes in said zone deleteriously. 
     
     
       7. The process of claim 6 wherein said HRT is in the range from 1-5 days, said SRT is in the range from 50-125 days, and said membranous filtration zone has a membrane area of at least 10 m 2 . 
     
     
       8. A continuous process for treating wastewater containing waste fluids from a metal-working plant in a bioreaction zone containing a suspension of biomass including live microorganisms adapted to biodegrade components of said waste fluids, comprising, (a) removing free oil and finely divided solids injurious to membrane means from delivered wastewater to provide a solids-depleted feed substantially free of free oil;   (b) maintaining the rate of flow of solids-depleted feed to said bioreaction zone essentially constant, said bioreaction zone containing bacterial cells greater than 0.5 μm diameter;   (c) pumping an aqueous suspension from said bioreaction zone to a membranous filtration zone from which permeate is recovered at a rate greater than the rate of solids-depleted feed;   (d) flowing said suspension through said membranous filtration zone at a velocity and pressure sufficient to maintain a predetermined membrane flux in the membranous filtration zone, at which flux essentially no solids are retained on the surface of membrane in the filtration zone;   (e) separating permeate from a solids-containing concentrate, said permeate being essentially free of molecules having an effective diameter greater than 0.5 μm;   (f) flowing said solids-containing concentrate as a concentrate recycle from said membranous filtration zone into the bioreaction zone;   (g) returning to said bioreaction zone from 0.1 but less than 0.5 part by volume of permeate generated;   (h) recovering the remainder of the permeate not returned to said bioreaction zone as treated effluent; and,   (i) periodically withdrawing a minor proportion by volume of said concentrate recycle to remove recalcitrant and biological solids in concentrated form.   
     
     
       9. The process of claim 8 including maintaining said bioreaction zone with a solids retention time (SRT) in the range from 30 to 150 days, and a hydraulic retention time (HRT) in the range from 1 to 5 days. 
     
     
       10. The process of claim 9 wherein said finely divided solids are greater than about 106 μm in size. 
     
     
       11. The process of claim 10 wherein said waste fluids comprise synthetic fluids, fats, oils and greases used in machining operations in said metal-working plant; said minor portion of permeate recycled is in the range from 0.1% to 30% by volume of the permeate withdrawn; and said permeate withdrawn as effluent is in the range from 0.5% to 3% by volume of said suspension of biomass withdrawn from said bioreaction zone. 
     
     
       12. The process of claim 11 comprising removing said effluent having at least the following specifications: Chemical Oxygen Demand, COD<450 mg/L;   Biological Oxygen Demand, BOD 5  <25 mg/L;   Total suspended solids, TSS<10 mg/L;   Total FOG<25 mg/L; and NH 3  --N<1.0 mg/L; wherein FOG represents fats, oils and greases.     
     
     
       13. The process of claim 12 wherein said membranous filtration zone is operated at a pressure in the range from 170 kPa to 1035 kPa gauge, insufficient to affect membranes in said zone deleteriously. 
     
     
       14. The process of claim 13 wherein said HRT is in the range from 1-5 days, said SRT is in the range from 50-125 days, and said membranous filtration zone has a membrane area of at least 10 m 2 . 
     
     
       15. A wastewater treatment system operatively connected between a source of wastewater containing waste fluids from a metal-working plant and an outlet for treated effluent, comprising, (a) a vessel large enough to hold said wastewater delivered at a variable rate of flow and to equalize the rates of flow to a predetermined essentially constant rate removed from said vessel;   (b) means to remove free oil from said wastewater to provide a skimmed wastewater;   (c) filtration means having a mesh size small enough to remove finely divided solids injurious to membrane means and provide a solids-depleted feed;   (d) a bioreactor means in which is confined an essentially constant volume of a reaction mass with means for aerating skimmed and solids-depleted feed, and for contacting said feed with waste-degrading microorganisms mixed therein;   (e) pump means for withdrawing a suspension of biomass from said bioreactor at an essentially constant rate of flow, and impelling said suspension at elevated pressure;   (f) membrane filtration means in direct, open flow-receiving communication with said pump means, said membrane filtration means including plural membrane elements adapted to separate said suspended solids in a concentrate stream, from water permeate which is essentially free of solids, and means to duct said permeate away from said system;   (g) means to return said concentrate bioreactor means; and,   (h) means to return a minor portion of said permeate to said bioreactor means.